Melatonin is a hormone produced by the pineal gland. It helps regulate the natural sleep-wake cycle.
Light affects how much melatonin your body produces. During the shorter days of winter, your body may make melatonin either earlier or later in the day than usual. This can lead to symptoms of seasonal affective disorder (SAD).
Melatonin supplements may be used to treat SAD, jet lag, and insomnia.